Is this parka warm enough for below-freezing weather?

Yes. The down fill is the main insulation and the shell blocks wind, so it handles sub-zero city cold well when worn over a sweater. In wet, windy conditions the down is the weak point once moisture gets in.

Can I wear it in heavy rain?

Light rain and snow are fine, but the 5K/5K shell is entry-level waterproofing. Sustained heavy rain will eventually soak through, and wet down clumps and loses loft.

How does it compare with a synthetic insulated parka?

Down is warmer for its weight and packs smaller, but synthetics keep insulating when wet. If you face wet winters more than dry cold, a synthetic parka is the safer pick.

What should I wear underneath?

A base layer plus a midweight sweater is enough for most winter days. The parka is cut to layer, so a bulky fleece underneath may feel tight.

How should I wash and dry it?

Use a down-specific detergent and a front-loading machine, then tumble dry low with dryer balls until the fill is fully lofted. Never dry-clean down or store it compressed.